Two-speed transfer case with electronic torque modulation

ABSTRACT

The present invention provides a power transfer system for a four-wheel drive vehicle which permits the vehicle operator to choose between operation in conventional two-wheel drive and four-wheel drive modes or an &#34;on-demand&#34; four-wheel drive mode. The power transfer system is operable for disabling the on-demand four-wheel drive feature upon the vehicle operator subsequently shifting into either of the two-wheel drive and the four-wheel drive modes. The system includes a transfer mechanism having a first and second output members coupled to first and second sets of wheels, respectively. The transfer mechanism has a transfer clutch that is operable for transferring drive torque from the first output member to the second output member to establish the part-time four-wheel drive and on-demand modes. A coupling mechanism is provided for mechanically coupling the first and second output members under certain conditions.

BACKGROUND OF THE INVENTION

The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle as a function of various system and operator-initiated inputs.

In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing tractive power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. However, the vast majority of such part-time transfer cases include a mechanical "mode" shift mechanism which can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions.

Alternatively, it is known to use "on-demand" power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. In one typical application, a viscous transmission device, such as a viscous coupling, is located in the driveline between the driven and non-driven wheels to provide "on-demand" four-wheel drive operation. Modernly, it is known to incorporate the "on-demand" feature into a transfer case by replacing the mechanical mode shift mechanism with a multi-disc clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated state such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque "on-demand" to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an "on-demand" power transfer system is disclosed in U.S. Pat. No. 4,773,500 to Naito, et al wherein a hydraulically-actuated clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function the wheel speed difference (i.e., the wheel slip) between the front and rear wheels.

While numerous variations of such hydraulically-actuated "on-demand" power transfer systems are known, they are primarily intended for road use in passenger cars and not for off-road recreational or sport/utility vehicles. In addition, such conventional "on-demand" transfer cases are single-mode power transfer mechanisms which do not provide means for permitting the vehicle operator to deliberately shift between traditional two-wheel drive and part-time four-wheel drive modes of operation. Thus, the need exists to develop a transfer case incorporating an "on-demand" four-wheel drive feature which can be effectively over-ridden upon the vehicle operator selectively shifting the transfer case into either of a two-wheel drive mode or a part-time four-wheel drive mode.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to overcome the disadvantages associated with conventional four-wheel drive systems by providing a power transfer system having an "on-demand" four-wheel drive feature incorporated into a transfer case. The "on-demand" feature can be intentionally "over-ridden" upon the vehicle operator selectively shifting the transfer case into one of a two-wheel drive mode or a part-time four-wheel drive mode. Thus, the present invention offers the vehicle operator the option of selecting the specific drive mode best-suited for operating the motor vehicle during normal or adverse road conditions as well as for off-road recreational use.

Pursuant to a related object of the present invention, the power transfer system is operably installed between the driven and non-driven wheels of a motor vehicle and is adapted to establish at least three distinct operative drive modes. In a preferred form, the power transfer system includes a transfer case equipped with a clutch assembly for selectively transferring drive torque to the non-driven wheels, actuator means for actuating the clutch assembly, sensor means for detecting various dynamic and operational characteristics of the vehicle and generating sensor input signals indicative thereof, mode select means for permitting the vehicle operator to select one of the distinct operative drive modes and generating a mode signal indicative thereof, and controller means for controlling actuation of the actuator means in response to the sensor input signals and the mode signal. The mode select means permits the vehicle operator to shift the transfer case into one of a two-wheel drive mode, a part-time four-wheel drive mode, and an "on-demand" mode. When the two-wheel drive mode is selected, the clutch assembly is de-actuated such that all drive torque is delivered to the driven wheels. When the part-time four-wheel drive mode is selected, the clutch assembly is fully actuated into a "lock-up" condition for distributing the drive torque between the driven and non-driven wheels as dictated by the tractive force generated at each respective set of wheels. When the "on-demand" mode is selected, the actuated condition of the clutch assembly is controllably modulated as a function of the sensor input signals for automatically varying the amount of drive torque directed to the non-driven wheels. In a preferred form, the power transfer system automatically provides improved traction and steering control upon the occurrence of instantaneous and unanticipated traction loss at the driven wheels when operating in the "on-demand" mode.

A further object of the present invention is to supply one or more "operator-initiated" input signals to the controller means for further controlling "on-demand" operation of the power transfer system. Preferably, the operator-initiated input signals are indicative of the position of a movable control element (i.e., accelerator pedal, throttle position, steering wheel, brake pedal, etc.) and are used, in conjunction with the sensor input signals, for optimizing the amount of drive torque delivered to the non-driven wheels during operation in the "on-demand" mode.

According to yet a further object, the transfer case is equipped with a speed reduction mechanism for establishing a direct-drive ("High") speed range and a reduced-ratio drive ("Low") speed range. In addition, the actuator means is operable for shifting the speed reduction mechanism between its various speed ranges in addition to controlling actuation of the clutch assembly. Thus, the vehicle operator is capable of selectively shifting the power transfer system between a two-wheel high-range drive mode (2WH), a part-time four-wheel high-range drive mode (4WH), a part-time four-wheel low-range drive mode (4WL) and the "on-demand" mode.

As a related object, coupling means are provided for mechanically locking the non-driven wheels to the driven wheels in response to the speed reduction mechanism being shifted into the low speed range for establishing the part-time four-wheel low-range drive mode (4WL) with the clutch assembly in a non-actuated condition.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention will become apparent from analysis of the following written specification, the accompanying drawings and the appended claims in which:

FIG. 1 is a schematic representation of an exemplary four-wheel drive vehicle having the power transfer system of the present invention incorporated therein;

FIG. 2 is a cross-sectional view of a transfer case associated with the power transfer system and which includes a clutch assembly, a speed reduction mechanism, a clutch lock-out mechanism, a drive mechanism, and an electronically-controlled actuator;

FIG. 3 is an enlarge partial view of FIG. 2 showing the various components in greater detail;

FIG. 4 is a side view of a sector plate that is rotatably driven by the actuator for actuating the clutch assembly and speed reduction mechanism;

FIG. 5 is a block diagram of the control system for a preferred embodiment of the present invention;

FIGS. 6 and 7 are flow charts depicting a control sequence for the operations performed by the control system of FIG. 5;

FIG. 8 illustrates exemplary plots of relationships between wheel speed differential at various vehicle speed ranges and the electrical control signal supplied by the control system to the actuator for controlling the amount of torque transferred through the clutch assembly during "on-demand" operation;

FIG. 9 is an exemplary plot of a relationship between steering angle and a control characteristic used for modifying the speed differential;

FIG. 10 graphically illustrates the relationship of the electrical control signal with respect to the output force generated by the drive mechanism and the corresponding drive torque transferred through the clutch assembly to the non-driven wheels;

FIG. 11 is a side view of an alternative construction for a sector plate driven by the actuator for actuating the clutch assembly and speed reduction mechanism; and

FIG. 12 is a flow chart depicting a control sequence associated with the part-time transfer case of the present invention equipped with the sector plate of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention is directed to a power transfer system which is operably installed between the driven and non-driven wheels of a four-wheel drive vehicle and which includes means for permitting a vehicleoperator to select between a two-wheel drive mode, a part-time four-wheel drive mode, and an "on-demand" four-wheel drive mode. When shifted into the "on-demand" drive mode, the amount of power (i.e., drive torque) delivered to the non-driven wheels is controllably regulated in accordancewith various system and driver-initiated inputs for optimizing the tractivecharacteristics of the vehicle while concomitantly enhancing overall steering control.

Referring to FIG. 1 of the drawings, a drivetrain for a four-wheel drive vehicle is schematically shown interactively associated with a power transfer system 10 which incorporates the novel principles of the present invention. The motor vehicle drivetrain has a pair of front wheels 12 and rear wheels 14 both drivable from a source of power, such as an engine 16,through a transmission 18 which may be of either the manual or automatic type. In the particular embodiment shown, the drivetrain is a rear wheel drive system which incorporates a transfer case 20 operable to receive drive torque from engine 16 and transmission 18 for normally driving rear wheels 14 (i.e., the "driven" wheels) in the two-wheel drive mode of operation. However, it is to be understood that the specific orientation of the drivetrain is merely exemplary in nature and that the drivetrain could be reversed for normally driving front wheels 12 in the two-wheel drive mode.

Front wheels 12 and rear wheels 14 are shown connected at opposite ends of front and rear axle assemblies 22 and 24, respectively. As is known, a rear differential 26 is interconnected between rear axle assembly 24 and one end of a rear drive shaft 28, the opposite end of which is interconnected to a first output member 30 of transfer case 20. Similarly,front axle assembly 22 includes a front differential 32 that is coupled to one end of a front drive shaft 34, the opposite end of which is coupled toa second output member 36 of transfer case 20. As will be detailed hereinafter with greater specificity, transfer case 20 is equipped with anelectronically-controlled torque transfer arrangement for transferring drive torque to front wheels 12 (i.e., the "non-driven" wheels) in addition to rear wheels 14 for defining the part-time four-wheel drive mode and the "on-demand" drive mode.

According to a preferred embodiment of power transfer system 10, the torquetransfer arrangement includes a transfer clutch 38 that is operable for selectively transferring drive torque from first output member 30 to second output member 36, thereby delivering drive torque to front wheels 12. Power transfer system 10 further comprises actuator means 40 for actuating transfer clutch 38, first sensor means 42 for sensing specific dynamic and operational characteristics of the motor vehicle and generating sensor input signals indicative thereof, mode select means 44 for permitting the vehicle operator to select one of the drive modes and generate a mode signal indicative thereof, and controller means 46 for generating a control signal in response to the sensor input signals and the mode signal. The controller means 46 is further operable for controlling the amount of drive torque transfer through transfer clutch 38to second output member 36 by sending the control signal to actuator means 40. When the two-wheel drive mode is selected, all drive torque is delivered from first output member 30 to rear wheels 14 and transfer clutch 38 is maintained in a "non-actuated" condition. When the part-time four-wheel drive mode is selected, transfer clutch 38 is fully actuated into a "lock-up" condition such that second output member 36 is, in effect, rigidly coupled for driven rotation with first output member 30. Thus, in the part-time four-wheel drive mode, drive torque is distributed between front and rear wheels 12 and 14, respectively, as dictated by the tractive forces generated at each respective set of wheels. When the "on-demand" drive mode is selected, controller means 46 controls the degree of actuation of actuator means 40 for instantaneously varying the amount of drive torque directed to front wheels 12 through transfer clutch38 as a function of the sensor input signals. In addition, the ability to controllably modulate the actuated condition of transfer clutch also provides for superior handling and steering control by substantially minimizing the oversteer and understeer tendencies of the vehicle during acornering maneuver, such tendencies known to be commonly associated with two-wheel drive operation and part-time four-wheel drive operation, respectively. Other advantages associated with controllably modulating theactuated state of transfer clutch 38 will be detailed hereinafter. As is schematically shown, controller means 46 is also operable for illuminatinga visual display 48, located within the passenger compartment, for providing the vehicle operator with a visual indication of the operationalstatus of power transfer system 10.

Power transfer system 10 can additionally include second sensor means 50 for generating "operator-initiated" input signals that are indicative of the position of one or more movable control elements under the control of the vehicle operator. The operator-initiated input signals are used for establishing control characteristics which, in conjunction with the sensorinput signals, further regulate the torque distribution during "on-demand" operation. Transfer case 20 is also equipped with speed reduction means 52for selectively transferring drive torque from engine 16 and transmission 18 to first output member 30 either directly (i.e., "high-range") or at a reduced-ration (i.e., "low-range"). Preferably, actuator means 40 is operable for shifting speed reduction means 52 between its various speed ranges and actuating transfer clutch 38 in response to the mode signal generated by mode select means 44. Thus, the vehicle operator has the option of shifting power transfer system 10 between a two-wheel high-rangedrive mode, a part-time four-wheel high-range drive mode, a part-time four-wheel low-range drive mode, and the "on-demand" drive mode. Finally, clutch lock-out means 54 are provided for mechanically coupling second output member 36 to first output member 30 in response to speed reduction means 52 being shifted into the low speed range. Thus, when the part-time four-wheel low-range mode is selected, transfer clutch 38 is shifted to the "non-actuated" condition and speed reduction means 52 is shifted into the low-range speed while clutch lock-out means 54 mechanically couples second output member 36 to first output member 30.

With particular reference to FIGS. 2 and 3, a preferred construction for transfer case 20 will now be described. Transfer case 20 is shown to include a housing 56 formed by a series of modular sections that are suitably interconnected in a conventional manner. A transmission output shaft 58 couples transmission 18 to an input shaft 60 of transfer case 20 for supplying power thereto. Input shaft 60 has an input sun gear 62 formed integral therewith and which is operably associated with speed reduction means 52. More specifically, sun gear 62 is meshed with a plurality of planet gears 64 (one shown), with each planet gear 64 being rotatably journalled on a pinion shaft 66 supported from a planetary carrier 68. Planet gears 64 also mesh with an annulus gear 70 which is non-rotatably mounted to an inner surface of housing 56. In operation, rotation of input shaft 60 results in planetary carrier 68 rotating at a reduced speed relative to sun gear 62. Thus, speed reduction means 52 is shown as a planetary gear assembly that is operable for establishing the high-range and low-range speed ratios. However, it will be appreciated that the planetary gear assembly is merely exemplary of one suitable speedreduction mechanism for use in transfer case 20.

According to the particular embodiment shown, first output member 30 is an elongated mainshaft 72 which is aligned on the longitudinal axis of input shaft 60 and is supported for rotation relative thereto. To provide means for selectively coupling mainshaft 72 to the output of speed reduction means 52, a range collar 74 is retained for rotation with, and axial sliding movement on, mainshaft 72 by means of collar internal splines 76 engaged with external splines 78 formed on mainshaft 72. In addition, range collar 74 is formed with external clutch teeth 80 that are shown in FIG. 2 to be engaged with sun gear internal clutch teeth 82. In the FIG. 2position, drive torque is transferred without reduction from input shaft 60to mainshaft 72 via range collar 74, thereby establishing the direct-drive or "high-range" speed position, as indicated by the construction line "H".To provide means for driving mainshaft 72 at the reduced speed ratio, planetary carrier 68 includes internal clutch teeth 84 which are placed inmeshed driving engagement with range collar external clutch teeth 80 (FIG. 3) upon range collar 74 being slid rearwardly for defining the "low-range"speed position, as indicated by the construction line "L". It will be notedthat when range collar 74 is positioned between the high-range and low-range speed positions, its clutch teeth 80 are disengaged from both sun gear clutch teeth 82 and planetary carrier clutch teeth 84 for defining a "Neutral" position, as indicated by construction line "N". Withrange collar 74 in the Neutral position, no drive torque is transmitted from input shaft 60 to mainshaft 72 and, thus, no power is transmitted to the vehicle's rear wheels 14. Axial sliding movement of range collar 74 between the various speed positions is caused by axial movement of a rangefork 86. As will be described, such movement of range fork 86 is controlledby actuator means 40 in response to the mode signal delivered to controllermeans 46 via mode select means 44.

With continued reference to FIGS. 2 and 3, means are shown for transferringdrive torque from mainshaft 72 to front wheels 12 through transfer clutch 38. More specifically, a drive sprocket 88 is shown fixed for rotation on a tubular extension 90 of a rotatable drum assembly 91 that is associated with transfer clutch 38. Extension 90 is shown rotatably supported by one or more suitable bearing assemblies 92 on mainshaft 72. Drive sprocket 88 drivingly engages a chain 94 which is coupled to a lower driven sprocket 96. Driven sprocket 96 is coupled to, or an integral portion of, second output member 36 of part-time transfer case 20. Second output member 36 isshown as a front output shaft 98 which is supported for rotation within housing 56 by suitable bearing assemblies 100 and 102. As noted, front output shaft 98 is operably connected to the motor vehicle's front wheel 12 via front drive shaft 34.

Transfer clutch 38 is shown operably installed within transfer case 20 for transferring drive torque from mainshaft 72 to front output shaft 98. Preferably, transfer clutch 38 is a mechanically-actuated multi-plate clutch assembly that is arranged to concentrically surround a central portion of mainshaft 72. As noted, elongated extension 90 is fixedly secured to drive sprocket 88 so as to drive, or be driven by, front outputshaft 98 of transfer case 20. In a preferred form, rotatable drum assembly 91 concentrically surrounds a clutch hub 104 that is fixed (i.e., splined)to mainshaft 72 for rotation therewith. Drum assembly 91 has an outer cylindrical drum 106 which is enclosed at one end by a cover plate 108. Asseen, extension 90 is integral with cover plate 108. Thus, drum assembly 91and clutch hub 104 are capable of rotating relative to one another and forman internal chamber 112 therebetween. Disposed within internal chamber 112 are two sets of alternately interleaved friction clutch plates that are operable for transferring torque from mainshaft 72 and clutch hub 104 to drum assembly 101 and front output shaft 98 in response to a clutch "engagement" force applied thereto. One set of clutch plates, referred to as inner clutch plates 114, are mounted for rotation with clutch hub 104 while the second set of clutch plates, referred to as outer clutch plates 116, are mounted for rotation with drum assembly 91. In addition, a reaction plate 118 is mounted on the aft end of outer cylindrical drum 106for enclosing the interleaved clutch plates within chamber 112. Reaction plate 118 is rotatable with drum assembly 91 and yet is axially movable with respect to interleaved friction clutch plates 114 and 116. Thus, reaction plate 118 acts as a pressure plate for compressing the interleaved clutch plates so as to cause drive torque to be transferred through transfer clutch 38 as a function of the clutch engagement force exerted thereon by actuator means 40. In the embodiment shown, reaction plate 118 is journally supported on the forward end of a spacer sleeve 119which, in turn, is fixed (i.e., splined) to mainshaft 72.

To provide means for selectively moving range collar 74 between its variousspeed range positions and for controlling the magnitude of the clutch engagement force exerted on reaction plate 118, actuator means 40 comprises a mechanical drive mechanism 120 and an electrically-controlled actuator 122. In general, actuator 122 is a rotary-driven actuator, such as an electric gearmotor, which is operable for generating an output torque, the value of which varies as a function of the magnitude of the electrical control signal applied thereto by controller means 46. More specifically, drive mechanism 120 is interconnected to a rotary output member 124 of actuator 122 for changing the output torque into an axially-directed force for controlling axial movement of range collar 74 and the engagement force applied to reaction plate 118 of transfer clutch 38. As best seen from FIGS. 3 and 4, drive mechanism 120 includes a sectorplate 126 that is rotatably driven through a limited range of angular motion by output member 124 of actuator 122. Sector plate 126 is interconnected to a lever arm assembly 128 for exerting the clutch engagement force on reaction plate 118 in response to controlled rotation of sector plate 126. In addition, sector plate 126 is also interconnected to a shift fork assembly 130 for concurrently controlling the axial position of range fork 86 and, in turn, range collar 74 in response to such controlled rotation of sector plate 126. Shift fork assembly 130 is aspring-loaded mechanism supported for movement on a shift rail 132 and which is substantially similar to that described in commonly owned U.S. Pat. No. 4,529,080 to Dolan, the disclosure of which is expressly incorporated by reference herein. From FIG. 2, shift rail 132 is shown to have its opposite ends disposed in fore and aft sockets 134 and 136, respectively, formed in housing 56.

A rotatable actuating shaft 138 is supported from transfer case housing 56 for rotation about an axis 140. A first end of actuating shaft 138 is secured in an aperture 142 formed in sector plate 126, with its opposite end coupled to output member 124 of electrically-controlled actuator 122. Thus, actuating shaft 138 and sector plate 126 are rotated about axis 140 in response to the control signal generated by controller means 46. From FIG. 4 it can be seen that sector plate 126 may be rotated about axis 140 by actuator shaft 138 to any of four distinct sector positions as labelled "4WH", "2WH", "N" and "4WL". To control movement of range collar 74, sector plate 126 has an elongated range slot 146 formed therein into whicha range pin 148 extends. From FIG. 3, it can be seen that range pin 148 is fixed to a U-shaped bracket 150 of shift fork assembly 130 which is retained for sliding movement on shift rail 132. Range slot 146 is configured to cause the desired translational movement of bracket 150, range fork 86 and range collar 74 in response to bi-directional rotation of sector plate 126.

To control the engagement force exerted on reaction plate 118, sector plate126 further includes a elongated mode slot 152 into which a crowned roller 154 extends. Crowned roller 154 is fixed to a flange section 156 of a lever member 158 associated with lever arm assembly 128. Lever member 158 also includes a generally Y-shaped or forked section 160 which is bifurcated to surround mainshaft 72 and clutch hub 90. The bifurcated endsof forked section 160 are retained for pivotal movement on a transverse rail 162, the ends of which are retained in suitable sockets (not shown) formed in housing 56. In general, the contour of mode slot 152 is configured to cause pivotable movement of lever member 158 in response to rotation of sector plate 126 for controlling the clutch engagement force exerted on reaction plate 118 of transfer clutch 38. Moreover, a pluralityof circumferentially-spaced buttons 164 are secured to a front surface of forked section 160 and are adapted to apply the clutch engagement force toreaction plate 118 for compressing the interleaved clutch plates 114 and 116 via a suitable thrust mechanism. Preferably, the thrust mechanism includes an annular ring-like apply plate 166 positioned intermediate reaction plate 118 and lever member 158, and a thrust bearing/washer arrangement 168 interposed between apply plate 166 and reaction plate 118 for allowing rotation of second cover plate 118 with respect to apply plate 166.

In view of the incorporation of speed reduction means 52 into transfer case20, power transfer system 10 is operable for establishing at least five distinct operational modes, namely a two-wheel high-range drive mode (2WH), a part-time four-wheel high-range drive mode (4WH), a part-time four-wheel low-range drive mode (4WL), the "on-demand" drive mode and the Neutral (N) mode. As described, the particular operational mode selected is established by the position of crowned roller 154 in mode slot 152 and the position of range pin 148 in range slot 146 in response to the angularposition of sector plate 126. In operation, the vehicle operator selects the desired mode via mode select means 44 which, in turn, signals controller means 46 of the selection. Thereafter, controller means 46 generates an electrical control signal that is applied to actuator 122 forcontrolling the rotated position of sector plate 126. Moreover, for each ofthe 2WH, 4WH, N and 4WL operative drive modes, sector plate 126 is controllably rotated to the predefined sector position. However, when shifted into the "on-demand" drive mode, power transfer system 10 is operable for modulating the clutch engagement force applied to transfer clutch 38 as a function of various system and operator initiated inputs.

With particular reference to FIG. 4, means are provided for coordinating the axial movement of shift fork assembly 130 and the pivotal movement of lever arm assembly 128 upon rotation of sector plate 126 between the various sector positions for establishing the desired combination of drivemodes and speed ranges. In general, the contour of range slot 146 is defined by first and second cam pathways 172 and 174, respectively, which respectively correspond to third and fourth cam pathways 176 and 178, respectively, associated with mode slot 152. In the sector position shown,crowned roller 154 is positioned within mode slot 152 in close proximity tothe terminal end of third cam pathway 176 for establishing a four-wheel drive (4WD) position. With crowned roller 154 in the four-wheel drive (4WD) position, lever arm assembly 128 exerts a maximum clutch engagement force on reaction plate 118 such that transfer clutch 38 is considered to be operating in a fully "actuated" or "locked-up" condition. As such, drive torque is transferred from mainshaft 72 and clutch hub 104 through the interleaved clutch plates to drum assembly 91 and, in turn, to front output shaft 98. In addition, range pin 148 is shown positioned within range slot 146 in close proximity to the terminal end of first cam pathways 172 for positioning range collar 74 in the high-range ("H") speedposition. Accordingly, when mode select means 44 signals that the vehicle operator has selected operation in the part-time four-wheel high-range drive mode, controller means 46 sends a control signal to actuator 122 forgenerating sufficient output torque to rotate sector plate 126 to the 4WH sector position.

As sector plate 126 is caused to rotate about axis 140 in a first (i.e., counterclockwise) direction from the position shown, the contour of third cam pathway 176 causes axial displacement of crowned roller 154 toward a two-wheel drive (2WD) position. Such movement of crowned roller 154 causesconcurrent pivotable movement of lever arm assembly 128 which results in a proportional decrease in the clutch engagement force being exerted on reaction plate 118. Moreover, once crowned roller 154 is in the two-wheel drive (2WD) position, lever arm assembly 128 does not exert an engagement force on reaction plate 118 which is sufficient to transfer drive torque through transfer clutch 38, whereby transfer clutch 38 is considered to bein a "non-actuated" condition. Concurrent with such movement of crowned roller 154 in mode slot 152 toward the two-wheel drive (2WD) position, range pin 148 is guided within first cam pathway 172 of range slot 146 formaintaining range collar 74 in the high-range (H) speed position. As such, when mode select means 44 signals that the vehicle operator has selected the two-wheel high-range drive mode, controller means 46 sends a control signal to actuator 122 for causing sector plate 126 to rotate to the 2WH sector position. As will be detailed, when the "on-demand" mode is selected, actuator 122 is actuated in accordance with specific predefined relationships established in response to the current value of the sensor input signals for rotatably driving sector plate 126 to any position between the 2WH and 4WH sector positions. Thus, the amount of torque transferred through transfer clutch 38 is proportional to the clutch engagement force, the value of which is determined by the particular position of crowned roller 154 between the two-wheel drive (2WD) position and the four-wheel drive (4WD) position that is established upon controlled rotation of sector plate 126.

Power transfer system 10 is also operable to permit transfer case 20 to be shifted into the "Neutral" mode and the part-time four-wheel low-range drive mode. More particularly, upon mode select means 44 signalling selection of either mode, actuator is controllably actuated for causing sector plate 126 to rotate in the first direction for concurrently guidingcrowned roller 154 within fourth cam pathway 178 of mode slot 152 and rangepin 148 within second cam pathway 174 of range slot 146. The contour of fourth cam pathway 178 is designed to retain crowned roller 154 in the two-wheel drive (2WD) position, whereby transfer clutch 38 is maintained in the "non-actuated" condition. However, range pin 148 is axially displaced due to the contour of second cam pathway 174 for sequentially shifting range collar 74 from the high-range (H) speed position into the Neutral (N) position or the low-range (L) speed position. Upon movement ofrange collar 74 from the high-range (H) speed position toward the Neutral (N) position, the aft end of range collar 74 abuts a forward end of a modesleeve 180 that is associated with lock-out clutch means 54, and shown in FIG. 2 positioned in an "unlocked" position. Mode sleeve 180 is retained for rotation with, and axially sliding movement on, mainshaft 72 by means of collar splines 182 engaged with mainshaft external splines 78. In addition, mode sleeve 180 includes internal clutch teeth 184 that are shown in FIG. 2 disengaged from external clutch teeth 186 of a dog sleeve 188 that is fixed (i.e., splined) to elongated extension 90 of drum assembly 91. Moreover, mode sleeve 180 is normally biased to the "unlocked" position by a mode return spring, such as compression spring 190. Thereafter, continued rearward movement of range collar 74 forcibly urges mode sleeve 180 to move rearwardly, in opposition to the biasing of return spring 190, until clutch teeth 184 are slid into driving meshed engagement with clutch teeth 186 on dog sleeve 188 (FIG. 3) when range collar 74 is in its low-range (L) speed position. In such a "lock-out" position, drive sprocket 88 is mechanically coupled to mainshaft 72 for establishing the part-time four-wheel low-range drive mode with transfer clutch 38 in its non-actuated condition. Thus, during "range" shifting of the planetary gear assembly, sector plate 126 is configured to maintain transfer clutch 38 in its non-actuated condition to eliminate the possibility of over-loading transfer clutch 38.

In its most basic sense, the "on-demand" mode is provided for automaticallyand instantaneously transferring drive torque to front wheels 12 during theoccurrence of slippage of rear wheel 14 that are typically associated with low tractive road conditions. Once the "on-demand" mode is selected, powertransfer system 10 functions to continuously monitor and regulate the "on-demand" operation in a manner that is independent of any deliberate action by the vehicle operator. As noted, the transfer of drive torque through transfer clutch 38 is preferably modulated while the planetary gear assembly is operating in the high-range speed for controlling the output of transfer clutch 38 between limits defined by the two-wheel high-range and part-time four-wheel high-range drive modes. Accordingly, the modulation range is established between the limits of bi-directional sector rotation defined by movement of crowned roller 154 within third campathway 176 between the (2WD) and (4WD) positions while range pin 148 travels within first cam pathway 172. According to the embodiment disclosed, the magnitude of the clutch engagement force generated by leverarm assembly 128 and applied to transfer clutch 38 is proportional to the magnitude of the output torque generated by actuator 122 which, in turn, is proportional to the magnitude of the control signal (i.e., percentage duty cycle) applied to actuator 122. Thus, the amount of drive torque transferred through transfer clutch 38 to front output shaft 98 is also proportional to the magnitude of the control signal. As such, the distribution ratio of drive torque between front output shaft 98 and mainshaft 72 of transfer case 20 may be selectively varied as a function of changes in the magnitude of the control signal for optimizing the tractive performance characteristics associated with operation in the "on-demand" four-wheel drive mode.

With particular reference now to FIG. 5, a block diagram of a control system associated with power transfer system 10 is shown. Preferably, controller means 46 is an electronic control module 192 having signal processing and information storage capabilities. In addition, first sensormeans 42 is shown as a group of various "system" sensors that are provided for detecting and signaling specific dynamic and operational characteristics of the motor vehicle. The input signals generated by the "systems" sensor group are delivered to electronic control module 192. Preferably, these sensors include a front speed sensor 194 for sensing therotational speed (n_(F)) of front drive shaft 34, a rear speed sensor 196for sensing the rotational speed (n_(R)) of rear drive shaft 28, a vehicle speed sensor 198 for sensing a vehicle speed (V), an ignition switch 200 for signalling the operational status of the vehicle, and a battery input 202 for powering electronic control module 192. In vehicles equipped with an anti-lock brake system (ABS), the original equipment ABS sensors provided at each wheel can be used for determining an "average" front drive shaft speed and rear drive shaft speed. Alternatively, front and rear speed sensors 194 and 196, respectively, can be arranged for directly measuring the speed of front output shaft 98 and mainshaft 72, respectively. Moreover, it is possible for vehicle speed sensor 198 to be eliminated with the vehicle speed signal (V) being computed from the frontrotational speed signal (n_(F)) generated by front speed sensor 194. However, it is to be understood that any suitable speed sensing arrangement capable of generating a signal indicative of the rotational speed of a shaft is fairly within the scope of the present invention.

The control system also illustrates the use of various "operator-initiated"inputs, as generally categorized by second sensor means 50. These inputs include a brake sensor 204 for sensing when the vehicle operator is applying the brakes, a gear position sensor 206 for sensing a gear position of transmission 18, a steering angle sensor 208 for detecting themagnitude of a steering angle (Φ), and an accelerator sensor for sensing an accelerating condition of the vehicle. Preferably, the accelerator sensor is a throttle position sensor 210 for sensing the degree of opening of a throttle valve associated with engine 16 or for sensing the degree of depression of an accelerator pedal, and is operable to produce a throttle position signal (A). The operator-initiated input signals are delivered to control module 192 where they are used, in conjunction with the system input signals, to further control "on-demand" operation.

To provide means for the vehicle operator to shift power transfer system 10into one of the available operational modes, mode select means 44 can take the form of any mode selector device which is under the control of the vehicle operator for generating a mode signal indicative of the specific mode selected. In one form, the mode selector device may be an array of dash-mounted push button switches. Alternatively, the mode selector devicemay be a manually-operably shift lever sequentially movable between a plurality positions corresponding to the available operational modes which, in conjunction with a suitable electrical switch arrangement, generates a mode signal indicating the mode selected. In either form, modeselect means 44 offers the vehicle operator the option of deliberately choosing between the part-time and on-demand operative drive modes.

With reference now to FIG. 6, a control sequence for the selection and the subsequent automatic control of the "on-demand" drive mode is shown in a flow chart. In general, the flow chart represents a sequence of operationsperformed by electronic control module 192 which are diagrammatically shownin block form. When mode select means 44 signals selection of the "on-demand" mode, as indicated by block 212, a succession of control stepsare repeated for selecting the value of the control signal to be applied toelectric actuator 122 in accordance with various predefined relationships between the current value of a front and rear wheel speed differential (ΔN) and vehicle speed (V), as modified by the steering angle (Φ) and other operator-initiated inputs. However, if any other mode isselected, then the control sequence jumps to a part-time routine, as indicated by block 214 and which is set forth in greater detail in FIG. 7.Block 216 is representative of the control step in which the current value of the rotational speed of front drive shaft 34 (n_(F)) and rear drive shaft 28 (n_(R)) are read. Block 218 indicates the step of reading the value of steering angle (Φ) as detected by steering angle sensor 208. Block 220 represents the operation of selecting a control characteristic (C) in accordance with the steering angle (Φ). FIG. 9 illustrates a plot of an exemplary relationship, which may be stored as a look-up table or computed from an arithmetic equation in control module 192, which correlates the control characteristic (C) as a linear function of the detected steering angle (Φ). Next, block 222 represents the step of calculating a speed differential (ΔN) according to the equation

    ΔN=n.sub.R -n.sub.F +C

Blocks 224 and 226 indicate the steps of reading the current value of the vehicle speed (V) as detected by vehicle speed sensor 198 and the throttleposition (A) as detected by throttle position sensor 210, respectively. As shown in block 228, control mode 192 determines whether the vehicle speed (V) exceeds a predefined threshold value (V_(T)) such as, for example, 5mph. If the vehicle speed is less than the threshold value (V_(T)), a second determination is made (block 230) as to whether the value of the throttle position (A) exceeds a predefined threshold value (A_(T)) such as, for example, a 50% accelerator pedal depression angle. If the vehicle speed (V) is less than its threshold value (V_(T)) and the throttle position (A) exceeds its threshold value (A_(T)), then the magnitude (i.e., percentage of duty cycle) of the electrical control signal is set as a preset value, such as 30% duty cycle, as indicated by block 232. In this manner, power transfer system 10 is adapted to transfer torque to front wheels 12 in response to acceleration at low vehicle speeds to inhibit wheel slip. However, if the value of the throttle position (A) is less than its threshold value (A_(T)), then the magnitude of the duty cycle for the control signal is set in accordance with predefined relationships between the speed differential signal (ΔN) and vehiclespeed (V), as indicated by block 234. Block 236 represents the step of outputting the electrical control signal to actuator 122 for developing the desired amount of torque transfer, if any, across transfer clutch 38. As shown in block 238, a timer circuit within control module 192 is actuated simultaneously with energization of actuator 122 for maintaining such energization for a predetermined time period (T). Once the period of energization (t) equals the predetermined time period (T) (or t≧T),control module 192 repeats the control routine.

To enhance steering control, block 240 is indicative of the control step between steps 228 and 234 for determining whether the vehicle operator is applying the brakes when the vehicle speed (V) is greater than the threshold value (V_(T)). Accordingly, if the vehicle operator is attempting to stop the vehicle, by applying the brakes (as sensed by brakesensor 204) during an occurrence of a low traction road condition an the vehicle speed (V) is greater than the predefined threshold (V_(T)), thencontrol module 192 sets the magnitude of the control signal sent to actuator 122 to zero (block 242) for de-actuating transfer clutch 38 and disabling the "on-demand" drive feature. This control sequence prevents simultaneous braking and "on-demand" four-wheel drive operation for providing the vehicle operator with greater steering and braking control. However, during the occurrence of a low traction condition when brake sensor 204 signals control module 192 that the vehicle operator is not applying the brakes, electronic control module 192 automatically energizesactuator 122 (block 236) for actuating transfer clutch 38 in accordance with the relationships generally denoted by block 234.

With particular reference to FIG. 8, a set of exemplary plots used for establishing the magnitude of the duty cycle in response to the current value of the speed differential (ΔN) and vehicle speed (V) during "on-demand" operation, as diagrammatically referred to by block 234 in FIG. 6, will now be detailed. As seen, power transfer system 10 is programmed to linearly correlate the percentage duty cycle of the control signal to be applied to actuator 122 to a range of speed differential (ΔN) values. In general, the percentage duty cycle for the control signal increases linearly from a minimum actuation value (Y%) to a maximumactuation value (100%) as the value of the speed differential (ΔN), within a particular vehicle speed range, increases from a minimum speed differential limit to a maximum speed differential limit (X). As such, when the value of the speed differential (ΔN) is less than the minimum speed differential limit, no drive torque is transferred through transfer clutch 38 to front output shaft 98 and the vehicle operates in anotherwise normal two-wheel drive mode. However, when the value of the speeddifferential (ΔN) exceeds the minimum differential limit, "on-demand"four-wheel drive operation is established by supplying a control signal to actuator 122 having a duty cycle value greater than (Y%). Thus, the minimum actuation duty cycle (Y%) for the control signal correlates to thepoint at which frictional engagement between interleaved clutch plates 114 and 116 results in the delivery of a portion of the total drive torque to front output shaft 98 of transfer case 20 for initiating four-wheel drive operation in the "on-demand" mode.

Preferably, the portion of the total drive torque transferred through transfer clutch 38 to front output shaft 98 increases substantially linearly as the magnitude of the duty cycle for the control signal increases from the minimum actuation value (Y%) to the maximum actuation value (100%). Preferably, the maximum value (X) of the speed differential (ΔN) correlates to the maximum actuation duty cycle (100%) at which point the maximum clutch engagement force is generated for completely locking-up clutch plates 114 and 116. During "on-demand" operation, a reduction in the magnitude of the control signal sent to actuator 122 willresult in actuator output member 124 being back-driven due to the clutch engagement load exerted by lever arm assembly 128 on sector plate 126. As such, a zero control signal will back-drive sector plate 126 to the two-wheel high-range (2WH) position. Alternatively, the direction of driven rotation of actuator output member 124 may be reversed until the desired clutch engagement force is established. As best seen from FIG. 10,an exemplary linear relationship between the magnitude of the duty cycle supplied to actuator 122 and the clutch engagement force generated and, inturn, the amount of torque delivered across transfer clutch 38 is shown.

When the vehicle operator selects a operational mode via mode select means 44 other than the "on-demand" mode, control module 192 is adapted to supply a control signal to actuator 122 for causing sector plate 126 to rotate to the particular sector position corresponding to the drive mode selected. FIG. 7 illustrates a flow chart representing an exemplary sequence of operations preformed by control module 192, herebefore referred to as part-time routine 214. In particular, blocks 244, 246, 248 and 250 represent the sequential steps of determining which of the two-wheel high-range drive mode (2WH), the part-time four-wheel high-rangedrive mode (4WH), the Neutral mode (N) or the part-time four-wheel low-range (4WL) drive mode, respectively, was signalled by mode select means 44. If the two-wheel high-range drive mode (2WH) was selected, block252 indicates the step of determining whether the prior mode was the part-time four-wheel high-range drive mode (4WH). If yes, block 254 represents the step of control module 192 sending a control signal to actuator 122 for rotating sector plate 126 in the first direction to its 2WH sector position. If no, block 256 indicates the operation of rotating sector plate 126 in the second direction to its 2WH sector position.

If block 246 indicates that the part-time four-wheel high-range drive mode (4WH) was selected, then control module 192 sends the maximal value of thecontrol signal to actuator 122 for causing sector plate 126 to rotate in the second direction to its 4WH sector position, as indicated by block 258. As noted, when the part-time four-wheel high-range drive mode is selected, transfer clutch 38 is fully actuated into a maximum torque transfer "lock-up" condition. If block 248 indicates that the Neutral mode(N) was selected, step 260 determines if the prior mode was either of the two-wheel high-range drive mode (2WH) or part-time four-wheel high-range drive mode (4WH). If yes, block 262 indicates the step of controllably actuating actuator 122 for rotating sector plate 126 in the first direction to its N sector position. If no, block 264 indicates the step ofactuating actuator 122 to rotate sector plate 126 in the second direction to its N sector position. Finally, if the part-time four-wheel low-range drive mode (4WL) is selected (block 250), then actuator 122 is actuated for rotating sector plate 126 in the first direction to its 4WL sector position, as indicated by block 266. As noted, rotation of sector plate 126 to the 4WL sector position results in mode sleeve 180 being forcibly urged into the "lock-out" position concurrent with transfer clutch 38 being returned to a non-actuated condition. As is common to most part-timesystems, it is contemplated that power transfer system 10 will inhibit completion of the shift into either of the Neutral mode or the part-time four-wheel low-range drive mode unless the vehicle speed (V) is less than the threshold value (V_(T)) and transmission 18 is in a Neutral gear position, as indicated by gear position sensor 206.

The present invention also incorporates means for maintaining the selected mode upon power interruption to actuator 122. To this end, a brake 270, schematically shown in FIGS. 2 and 5, is provided. Brake 270 is an electrically-controlled spring-applied device that is operable in a "power-off" condition for braking inertial loads (i.e., rotation) exerted on output member 124 of actuator 122 and, in turn, actuation shaft 138. Inoperation, control module 192 delivers an electrical signal to brake 270 tomaintain it in a released or "power-on" condition. During controlled rotation of sector 126, brake 270 is maintained in its released "power-on"condition. However, upon interruption of power to brake 270, brake torque is generated for inhibiting rotation of shaft 138. Thus, once sector plate126 is positioned into one of its part-time positions, power to brake 270 is interrupted for positively retaining sector plate 126 in the rotated position. Thereafter, power to actuator 122 can be interrupted to minimizeits on-time service requirements.

With particular reference to FIGS. 11 and 12, an alternative embodiment of a mechanical drive mechanism 120' is disclosed which can be readily incorporated into transfer case 20 of power transfer system 10. In general, drive mechanism 120' includes a sector plate 280 that is similar to sector plate 126 with the exception that the profile of a mode slot 282has been modified for reordering the sequence of drive modes established upon rotation of sector plate 280. However, power transfer system 10 is still operable for establishing the five distinct operational modes, namely the two-wheel high-range drive mode, the part-time four-wheel high-range drive mode, the part-time four-wheel low-range drive mode, the "on-demand" drive mode and the Neutral mode. As before, the particular operational mode selected is established by the position of crowned roller154 in mode slot 282 and the position of range pin 148 in range slot 146 inresponse to the angular position of sector plate 280. In operation, the vehicle operator selects the desired mode via mode select means 44 which, in turn, signals controller means 46 of the selection. Thereafter, controller means 46 generates an electrical control signal that is appliedto actuator 122 for controlling the rotated position of sector plate 280. However, when shifted into the "on-demand" drive mode, power transfer system 10 is operable for modulating the amount of drive torque delivered to front output shaft 98 through transfer clutch 38 as a function of various system and operator initiated inputs. For purposes of clarity, like numbers are used to designate those components of drive mechanism 120' and sector plate 280 that are similar to components previously described.

With particular reference to FIG. 11, means are provided for coordinating the axial movement of shift fork assembly 130 and pivotal movement of lever arm assembly 128 upon rotation of sector plate 280 between the various sector positions for establishing the desired combination of drivemodes and speed ranges. In general, the contour of range slot 146 is defined by first and second cam pathways 172 and 174, respectively, which respectively correspond to third and fourth cam pathways 284 and 286, respectively, associated with mode slot 282. In the sector position shown,crowned roller 154 is positioned within mode slot 282 in close proximity tothe terminal end of third cam pathway 284 for establishing a two-wheel drive (2WD) position. In the two-wheel drive (2WD) position, lever arm assembly 128 does not exert an engagement force on apply plate 166 which is sufficient to transfer drive torque through transfer clutch 38, wherebytransfer clutch 38 is considered to be in the "non-actuated" condition. In addition, range pin 148 is shown positioned within range slot 146 in closeproximity to the terminal end of first cam pathways 172 for positioning range collar 74 in the high-range ("H") speed position. As such, when sector plate 280 is in the 2WH sector position shown, transfer case 20 is operating in a two-wheel high-range drive mode.

As sector plate 280 is caused to rotate about axis 140 in the first direction from the position shown, the contour of third cam pathway 284 causes axial displacement of crowned roller 154 toward a four-wheel drive (4WD) position. Such movement of crowned roller 154 causes concurrent pivotable movement of lever arm assembly 128 for exerting a clutch engagement force on apply plate 166, whereby drive torque is transferred from mainshaft 72 through clutch hub 104 and interleaved clutch plates 114and 116 to drum assembly 91 and, in turn, to front output shaft 98. In the four-wheel drive (4WD) position, lever arm assembly 128 exerts a maximum clutch engagement force such that transfer clutch 38 is considered to be operating in the fully "actuated" condition. Concurrent with such movementof crowned roller 154 toward the four-wheel drive (4WD) position, range pin148 is guided within first cam pathway 172 of range slot 146 for maintaining range collar 74 in the high-range (H) speed position, thereby establishing a four-wheel high-range drive mode. As will be appreciated, rotation of sector plate 280 in the opposite (i.e., clockwise) direction from the 4WH sector position is operable for proportionally decreasing theclutch engagement force exerted on transfer clutch 38 until transfer case 20 is returned to operation in the two-wheel high-range drive mode. As wasdetailed, when the "on-demand" mode is selected, actuator 122 is actuated in response to the specific predefined relationships established in accordance with the sensor input signals for rotatably driving sector plate 280 to any position between the 2WH and 4WH sector positions. Thus, the amount of torque transferred to front output shaft 98 through transferclutch 38 is proportional to the clutch engagement force, the value of which is determined by the particular position of crowned roller 154 between the two-wheel drive (2WD) position and the four-wheel drive (4WD) position that is established upon controlled rotation of sector plate 280.

Power transfer system 10 is also operable to permit transfer case 20 to be shifted into the "Neutral" mode and the part-time four-wheel low-range drive mode. More particularly, upon selection of either mode, sector plate280 is caused to rotate in the first direction for concurrently guiding crowned roller 154 within fourth cam pathway 286 of mode slot 282 and range pin 148 within second cam pathway 174 of range slot 146. The contourof fourth cam pathway 286 is designed to return crowned roller 154 to the two-wheel drive (2WD) position for returning transfer clutch 38 to the "non-actuated" condition. However, range pin 148 is axially displaced due to the contour of second cam pathway 174 for sequentially shifting range collar 74 from the high-range (H) speed position into the Neutral (N) position or the low-range (L) speed position. Thus, during "range" shifting of the planetary gear assembly, sector plate 280 is configured toreturn transfer clutch 38 to its non-actuated condition to eliminate the possibility of over-loading transfer clutch 38. As before, movement of range collar 74 to the low-range (L) speed position results in concurrent movement of mode sleeve 180 from its "unlocked" position into the "locked"position.

When the vehicle operator selects a operational mode via mode select means 44 other than the "on-demand" mode, control module 192 energizes actuator 122 for rotating sector plate 280 to the particular sector position which corresponds to the mode selected. FIG. 12 illustrates an exemplary flow chart representing a sequence of operations preformed by control module 192, and referred to as part-time routine 214'. In particular, blocks 244,246, 248 and 250 represent the sequential steps of determining which of thetwo-wheel high-range drive mode (2WH), the part-time four-wheel high-range drive mode (4WH), the Neutral mode (N) or the part-time four-wheel low-range (4WL) drive mode, respectively, was signalled by mode select means 44. If the two-wheel high-range drive mode (2WH) was selected, block300 indicates the operation of signalling actuator 122 to rotate sector plate 280 in the second direction to its 2WH sector position. If block 246indicates that the part-time four-wheel high-range drive mode (4WH) was selected, block 302 represents the step of determining whether the prior mode was the two-wheel high-range drive mode (2WH). If yes, block 304 represents the step of control module 192 sending a control signal to actuator 122 for rotating sector plate 280 in the first direction to its 4WH sector position. If no, block 306 indicates the operation of rotating sector plate 280 in the second direction to its 4WH sector position. If block 248 indicates that the Neutral mode (N) was selected, block 308 represents the step of determining whether the prior mode was either of the two-wheel high-range drive mode (2WH) or part-time four-wheel high-range drive mode (4WH). If yes, block 310 indicates the step of actuating actuator 122 for rotating sector plate 280 in the first direction to its N sector position. If no, block 312 indicates the step ofactuating actuator 122 to rotate sector plate 280 in the second direction to its N sector position. Finally, if the four-wheel low-range drive mode (4WL) is selected (block 250), then step block 314 indicates the step of rotating sector plate 280 in the first direction to its 4WL sector position.

The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined in the following claims. 

What is claimed is:
 1. A power transfer system for a motor vehicle having an engine and first and second sets of wheels, said power transfer system comprising:a transfer mechanism having an input member rotatably driven by the engine, a first output member interconnecting said input member to the first set of wheels for transmitting drive torque from the engine to the first set of wheels for defining a two-wheel drive mode, a second output member interconnected to the second set of wheels, a speed reduction mechanism driven by said input member for establishing a reduced-ratio low-range drive, a shift member movable between a first position for directly coupling said first output member to said input member for establishing a high-range drive and a second position for coupling said first output member to said speed reduction mechanism for establishing said low-range drive, a clutch assembly operable in an actuated state for selectively transmitting drive torque from said first output member to said second output member for establishing a four-wheel drive mode, coupling means for mechanically coupling said first output member to said second output member in response to movement of said shift member to said second position, and biasing means for urging said coupling means to a position de-coupled from said second output member upon movement of said shift member to said first position; sensor means for detecting dynamic and operational characteristics of the vehicle and generating sensor input signals in response thereto; mode select means for enabling a vehicle operator to intentional select one of a two-wheel high-range drive mode, a four-wheel high-range drive mode, a four-wheel low-range drive mode and an on-demand four-wheel high-range drive mode, said mode select means further operable for generating a mode signal indicative of the particular drive mode selected; coordinating means for coordinating movement of said shift member and said actuated state of said clutch assembly; actuator means for selectively moving said coordinating means; and controller means for controlling actuation of said actuator means in response to said mode signal, said controller means operable for causing said actuator means to move said coordinating means to a 2WH position to de-actuate said clutch assembly and move said shift member to said first position when said two-wheel high-range drive mode is selected, said controller means causing said actuator means to move said coordinating means to a 4WH position to fully actuate said clutch assembly and move said shift member to said first position when said four-wheel high-range drive mode is selected, said controller means causing said actuator means to move said coordinating means to a 4WL position to de-actuate said clutch assembly and move said shift member to said second position when said four-wheel low-range drive mode is selected, and said controller means causing said actuator means to modulate the position of said coordinating means between said 2WH and 4WH positions to vary the degree of actuation of said clutch assembly between said de-actuated and fully actuated conditions as a function of said sensor input signals for modulating the drive torque transmitted to the second set of wheels while maintaining said shift member in said first position when said on-demand mode is selected.
 2. The power transfer system of claim 1 further comprising second sensor means for generating a second sensor input signal indicative of a position of a movable control element under the control of said vehicle operator, said second sensor input signal being used in conjunction with said first sensor input signals for modifying the actuated condition of said clutch assembly during operation in said on-demand four-wheel drive mode.
 3. The power transfer system of claim 1 wherein said coordinating means comprises a drive mechanism for moving said shift member between said first and second positions, said drive mechanism further operable for exerting a clutch engagement force on a reaction member of said clutch assembly for actuating said clutch assembly in response thereto, and said actuator means further comprising an electrically-driven actuator interconnected to said drive mechanism for controlling movement of said drive mechanism in response to the magnitude of an electrical control signal supplied to said actuator from said controller means.
 4. The power transfer system of claim 3 wherein said reaction member is adapted to exert said clutch engagement force on said clutch assembly for varying the drive torque transmitted to said second output member in response to controlled movement of said drive mechanism.
 5. The power transfer system of claim 4 wherein said drive mechanism is movable between said 2WH position wherein said shift member is in said first position and a minimum clutch engagement force is exerted on said reaction member for de-actuating said clutch assembly, said 4WH position wherein said shift member is in said first position and a maximum clutch engagement force is exerted on said reaction member for fully actuating said clutch assembly, and said 4WL position wherein said shift member is in said second position and said minimum clutch engagement force is exerted on said reaction member.
 6. The power transfer system of claim 1 wherein said coordinating means is selectively rotatable between said positions in response to said mode signal, whereby when said mode signal indicates selection of said two-wheel high-range drive mode said control signal causes said actuator to rotate said coordinating means to said 2WH position, when said four-wheel high-range drive mode is selected said control signal causes said actuator to rotate said coordinating means to said 4WH position, and when said four-wheel low-range drive mode is selected said control signal causes said actuator to rotate said coordinating means to said 4WL position.
 7. The power transfer system of claim 1 wherein said first sensor means is operable for sensing the rotational speed of said first and second output members of said transfer mechanism, said controller means being operable to receive said input signals from said first sensor means and generate a speed differential signal in response thereto which is indicative of a speed differential between said first and second output members, and wherein said mode select means is a manually-operable mode select member for generating said mode signal indicative of the particular drive mode selected by said vehicle operator, said controller means being adapted to automatically actuate said actuator means when said speed differential signal exceeds a predetermined minimum value and said mode signal directs said power transfer system to operate in said on-demand four-wheel drive mode.
 8. The power transfer system of claim 7 wherein said controller means actuates said actuator means for linearly increasing the magnitude of the drive torque transferred through said clutch assembly to said second output member of said transfer mechanism in response to increasing values of said speed differential signal between said predetermined minimum value and a predetermined maximum value.
 9. The power transfer system of claim 8 further comprising second sensor means for generating a second sensor input signal indicative of a position of a movable control element under the control of said vehicle operator, said second sensor input signal being used in conjunction with said first sensor input signals for modifying the actuated condition of said clutch assembly during operation in said on-demand four-wheel drive mode.
 10. The power transfer system of claim 9 wherein said second sensor means includes a brake sensor for providing a brake signal to said controller means that indicates when said vehicle operator is applying the brakes, said controller means being operably for automatically de-actuating said clutch assembly during on-demand operation in response to application of the brakes.
 11. The power transfer system of claim 1 further comprising brake means that is be automatically actuated for inhibiting movement of said actuator means in response to interruption of power thereto.
 12. The power transfer system of claim 1 wherein upon selection of said on-demand mode, said controller means causes said actuator to modulate the position of said coordinating means between said 2WH position and said 4WH position in accordance with pre-established relationships based on said sensor input signals.
 13. The power transfer system of claim 12 wherein said first sensor means is operable for sensing the rotational speed of said first and second output members of said transfer mechanism, said controller means being operable to receive said input signals from said first sensor means and generate a speed differential signal in response thereto which is indicative of a speed differential between said first and second output members, and wherein said mode select means is a manually-operable mode select member for generating said mode signal indicative of the particular drive mode selected by said vehicle operator, said controller means being adapted to automatically actuate said actuator means when said speed differential signal exceeds a predetermined minimum value and said mode signal directs said power transfer system to operate in said on-demand four-wheel drive mode, whereby said clutch engagement force exerted by said reaction member on said clutch assembly transfers drive torque from said first output member to said second output member.
 14. The power transfer system of claim 13 wherein said controller means actuates said actuator means for linearly increasing the magnitude of the drive torque transferred through said clutch assembly to said second output member of said transfer mechanism in response to increasing values of said speed differential signal between said predetermined minimum value and a predetermined maximum value.
 15. In a vehicle having a first set of wheels and a second set of wheels adapted to be driven from a source of power, a power transfer system comprising:a transfer mechanism having an input member rotatably driven by the power source, a first output member interconnecting said input member to the first set of wheels for transmitting drive torque from the power source to the first set of wheels for defining a two-wheel drive mode, a second output member interconnected to the second set of wheels, a speed reduction mechanism driven by said input member for establishing a reduced-ratio low-range drive, a shift member movable between a first position for directly coupling said first output member to said input member for establishing a high-range drive and a second position for coupling said first output member to said speed reduction mechanism for establishing said low-range drive, a clutch assembly operable in an actuated state for selectively transmitting drive torque from said first output member to said second output member for establishing a four-wheel drive mode, coupling means for mechanically coupling said first output member to said second output member in response to movement of said shift member to said second position, and biasing means for urging said coupling means to a position de-coupled from said second output member upon movement of said shift member to said position; first drive means interconnecting said first output member of said transfer mechanism to the first set of wheels; second drive means interconnecting said second output member of said transfer mechanism to the second set of wheels; first sensor means for sensing the rotational speed of said first drive means and generating a first input signal indicative thereof; second sensor means for sensing the rotational speed of said second drive means and generating a second input signal indicative thereof; mode select means for enabling a vehicle operator to intentionally select one of a two-wheel high-range drive mode, a four-wheel high-range drive mode, a four-wheel low-range drive mode and an on-demand four-wheel high-range drive mode, said mode select means further operable for generating a mode signal indicative of the particular drive mode selected; coordinating means for coordinating movement of said shift member and said actuated state of said clutch assembly; actuator means for selectively moving said coordinating means; and controller means for receiving said first and second input signals and generating a speed differential signal in response thereto that is indicative of a speed differential between said first and second drive means, and said controller means controlling actuation of said actuator means in response to said mode signal for causing said actuator means to move said coordinating means to a 2WH position to de-actuate said clutch assembly and move said shift member to said first position when said two-wheel high-range drive mode is selected, said controller means causing said actuator means to move said coordinating means to a 4WH position to fully actuate said clutch assembly and move said shift member to said first position when said four-wheel high-range drive mode is selected, said controller means causing said actuator means to move said coordinating means to a 4WL position to de-actuate said clutch assembly and move said shift member to said second position and when said four-wheel low-range drive mode is selected, and said controller means causing said actuator means to modulate the position of said coordinating means between said 2WH and 4WH positions to vary the degree of actuation of said clutch assembly between said de-actuated and fully actuated conditions as a function of said sensor input signals for modulating the drive torque transmitted to the second set of wheels while maintaining said shift member in said first position when said on-demand mode is selected.
 16. The power transfer system of claim 15 wherein said first drive means comprises a rear axle assembly such that the motor vehicle is a rear wheel drive vehicle.
 17. The power transfer system of claim 15 wherein said clutch assembly is installed between said first and second outputs of said transfer mechanism, and wherein said controller means is an electronic control module having circuit means for processing said signals from each of said first and second sensor means and generating said speed differential signal in response thereto.
 18. The power transfer system of claim 17 wherein said controller means is operable to automatically modulate the magnitude of the drive torque transferred from said first output to said second output of said transfer mechanism as a linear function of changes in the value of said speed differential signal during operation in said on-demand drive mode.
 19. The power transfer system of claim 15 further comprising third sensor means for generating a third sensor input signal indicative of a position of a movable control element under the control of said vehicle operator, said third sensor input signal being used for modifying the actuated condition of said clutch assembly during operation in said on-demand drive mode.
 20. The power transfer system of claim 15 wherein said coordinating means comprises a drive mechanism for moving said shift member between said first and second positions, said drive mechanism is further operable for exerting a clutch engagement force on a reaction member of said clutch assembly for actuating said clutch assembly in response thereto, and said actuator means further comprising an electrically-driven actuator interconnected to said drive mechanism for controlling movement of said drive mechanism in response to the magnitude of an electrical control signal supplied to said actuator from said controller means.
 21. The power transfer system of claim 20 wherein said reaction member is adapted to exert said clutch engagement force on said clutch assembly for varying the drive torque transmitted to said second output member in response to controlled movement of said drive mechanism.
 22. The power transfer system of claim 21 wherein said drive mechanism is operable for coordinating the movement of said reaction member and said shift member to establish said various drive modes, and wherein said drive mechanism is adapted to cause movement of said coupling means in response to movement of said shift member.
 23. The power transfer system of claim 22 wherein said drive mechanism is movable between said 2WH position wherein said shift member is in said first position and a minimum clutch engagement force is exerted on said reaction member for de-actuating said clutch assembly, said 4WH position wherein said shift member is in said first position and a maximum clutch engagement force is exerted on said reaction member for fully actuating said clutch assembly, and said 4WL position wherein said shift member is in said second position and said minimum clutch engagement force is exerted on said reaction member.
 24. The power transfer system of claim 23 wherein said drive mechanism is selectively rotatable between said positions in response to said mode signal, whereby when said mode signal indicates selection of said two-wheel high-range drive mode said control signal causes said actuator to rotate said drive mechanism to said 2WH position, when said four-wheel high-range drive mode is selected said control signal causes said actuator to rotate said drive mechanism to said 4WH position, and when said four-wheel low-range drive mode is selected said control signal causes said actuator to rotate said drive mechanism to said 4WL position.
 25. The power transfer system of claim 24 wherein upon said mode signal indicating selection of said on-demand drive mode said actuator is actuated to modulate movement of said coordinating means between said two-wheel high-range position and said four-wheel high-range position as a function of said sensor input signals. 